The present invention relates to a device for sensing overloads on a lifting boom, such as used in cranes, and aerial lift platforms.
Lifting booms are provided on various types of apparatus, such as cranes and aerial lift platforms, which latter are characterized by providing a platform or basket for one or more workmen. It has been recognized that these lifting booms may be overloaded, due to a combination of weight, boom angle and boom extension. Thus, it is known that the greater the weight, the greater the angle of the boom to vertical and the greater the extension of the boom, in the case of an extensible boom, the greater will be the bending moment. Such bending moments deflect or strain the boom, and where excessive, boom failure will occur. Typical instances of boom failure occur when a weight is swung outwardly by movement of the boom, either by downward luffing or by extension, or when a load which is too great is attempted to be lifted with the boom near the horizontal position.
A variety of devices have been proposed in order to prevent boom overload and failure. In some instances, cams and sensors have been provided, connected either to an alarm signal or to a control of the boom, such as the electrical circuitry in the case of a boom in which movement is controlled through switches and the like. These have not been as facile as desirable, often requiring multiple cams, and being subject to wear. Another proposal which has been made is to sense the pressure of the hydraulic fluid in a cylinder connected with the boom, the hydraulic fluid pressure being related to the load on the boom. This arrangement, although widely used, is not as sensitive as is desired: that is, when a boom, while being moved, approaches a position in which the pressure in the hydraulic cylinder becomes excessive, a substantial amount of back travel, either retraction of an extensible boom, or lifting, or both, are required in order to reduce the pressure of the hydraulic fluid sufficiently to enable continued movement of the boom. Yet another proposal in the prior art has utilized transducers, such as strain gauges, for sensing the strain of the material comprising the boom. These arrangements, however, required a significant amount of electrical circuitry, thus adding to the cost, and requiring sophisticated and relatively expensive sensing equipment to sense the changes in the electrical characteristics of the strain gauge and/or transducers resulting from the strain or deflection of the boom material.
There has also been provided, in addition, an overload safety device for material handling mechanism which provided for the detection of an approaching overload condition by sensing deflection in a stressed member, and then automatically interrupting circuitry controlling the means causing the deflection-increasing movement of the load, while at the same time permitting actuation of means for deflection-decreasing movements. In this construction, a lever arm in the form of a hollow tube was welded directly to two lever arm mounts which were in turn welded to the boom, to either the top or side thereof. The spacing of the mounts was the width of the lever arm-beam, which width was apparently that which was required to avoid deflection of the lever arm-beam and consequently malfunction of the device. A switch bracket was pivotally connected to the outer end of the lever arm, being biased by a spring, and carrying at its outer end one or more micro-switches. A bracket welded to the boom carried adjustable set screws, in juxtaposition with the switch or switches. When the boom deflected, there was relative movement between the micro-switches carried on the bracket and lever arm, on the one hand, and the set screw and bracket on the other hand, resulting an actuation of the micro-switch or micro-switches, which thereupon changed the circuitry and prevented further movement toward the overload condition. While this construction avoids many of the noted deficiencies of other constructions, it was not as sensitive as desirable, appears to result in vibration of the lever arm and/or switch bracket, and further, by mounting the micro-switches on the lever arm and switch bracket, with an intervening spring, seems not to provide a construction which would give the same results on repeated usage. Otherwise stated, this construction may well produce results which vary from time to time, under the same conditions. Further, although a weather shield was disclosed on this construction, there could result a failure if the shield leaked, and the spring between the lever arm and the switch bracket rusted.